Two-step STI formation process

ABSTRACT

A method of forming an integrated circuit structure includes providing a semiconductor substrate; forming a first isolation region in the semiconductor substrate; after the step of forming the first isolation region, forming a metal-oxide-semiconductor (MOS) device at a surface of the semiconductor substrate, wherein the step of forming the MOS device comprises forming a source/drain region; and after the step of forming the MOS device, forming a second isolation region in the semiconductor substrate.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and moreparticularly to the formation of isolation structures for separatingintegrated circuits.

BACKGROUND

Modern integrated circuits are formed on the surfaces of semiconductorsubstrates, which are mostly silicon substrates. Semiconductor devicesare isolated from each other by isolation structures formed close to thesurface of the respective semiconductor substrates. The isolationstructures include field oxides and shallow trench isolation (STI)regions.

Field oxides are often formed using local oxidation of silicon (LOCOS).A typical formation process includes blanket forming a mask layer on asilicon substrate, and then patterning the mask layer to expose certainareas of the underlying silicon substrate. A thermal oxidation is thenperformed in an oxygen-containing environment to oxidize the exposedportions of the silicon substrate. The mask layer is then removed.

With the down-scaling of integrated circuits, STI regions areincreasingly used as the isolation structures. FIG. 1 illustrates a topview of an integrated circuit structure, which includesmetal-oxide-semiconductor (MOS) devices 2 and 12. MOS device 2 includesgate poly 4 formed over active region 6. MOS device 12 includes gatepoly 14 formed over active region 16. Active regions 6 and 16 areseparated from each other and from other devices by STI regions 8, whichinclude STI strips 8 ₁ parallel to the gate length direction(source-to-drain direction) of MOS devices 2 and 12, and STI strips 8 ₂parallel to the gate width direction.

The formation of STI regions 8 is typically performed before theformation of MOS devices. During the subsequent high-temperature processsteps, which may be performed at temperatures as high as about 700° C.,stresses are generated due to the different coefficients of thermalexpansion between STI regions 8 and active regions 6 and 16. STI regions8 thus apply stresses to active regions 6 and 16, affecting theperformance of MOS devices 2 and 12. In addition, the formation ofsource/drain regions requires dopant implantations. In the portions ofactive regions 6 and 16 near STI regions 8, dopant concentrations mayhave fluctuations due to the diffusion of the dopants into STI regions8.

To make the situation worse, typically, the width W1 of STI strips 8 ₁is greater than width W2 of STI strips 8 ₂. Voids are thus more likelyto be generated in STI regions 8 ₂. This causes the stresses generatedby STI regions 8 ₂ in the gate length direction to be adversely changed.Accordingly, what is needed in the art is a method for solving theabove-discussed problems.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method offorming an integrated circuit structure includes providing asemiconductor substrate; forming a first isolation region in thesemiconductor substrate; after the step of forming the first isolationregion, forming a metal-oxide-semiconductor (MOS) device at a surface ofthe semiconductor substrate, wherein the step of forming the MOS deviceincludes forming a source/drain region; and after the step of formingthe MOS device, forming a second isolation region in the semiconductorsubstrate.

In accordance with another aspect of the present invention, a method offorming an integrated circuit structure includes providing asemiconductor substrate; and forming a MOS device, which includesforming a gate stack on a top surface of the semiconductor substrate;forming a gate spacer on a sidewall of the gate stack; forming asource/drain region adjacent the gate spacer; and forming a silicideregion on the source/drain region. The method further includes etching aportion of the silicide region and a portion of the source/drain regionto form a trench, wherein the trench is spaced apart from an outer edgeof the gate spacer by a remaining portion of the source/drain region,and wherein the trench extends below a bottom of the source/drainregion; and forming an inter-layer dielectric (ILD) over the MOS device,wherein the ILD extends into the trench to form a shallow trenchisolation (STI) region.

In accordance with yet another aspect of the present invention, a methodof forming an integrated circuit structure includes providing asemiconductor substrate; forming a first STI region in the semiconductorsubstrate, wherein the first STI region has a first lengthwisedirection, and wherein the first STI region adjoins an active region;forming a first and a second gate electrode strip over the active regionand the first STI region, wherein the first and the second gateelectrodes are parallel to each other; forming a common source/drainregion in the active region and between the first and the second gateelectrodes; forming a silicide region on the common source/drain region;etching a portion of the silicide region and a portion of the commonsource/drain region to form a trench, wherein the trench has a secondlengthwise direction parallel to the first and the second gate electrodestrips; and forming an ILD having a top surface higher than top surfacesof the first and the second gate electrode strips, wherein the ILDextends into the trench to form a second STI region.

In accordance with yet another aspect of the present invention, anintegrated circuit structure includes a semiconductor substrateincluding an active region; a first STI region adjoining a first side ofthe active region; a gate electrode over the active region and the firstSTI region; a source/drain region adjacent the gate electrode; a trenchin the semiconductor substrate and adjoining a second side of the activeregion, wherein the trench has a bottom no higher than a bottom of thesource/drain region; and an ILD extending from over the gate electrodeto inside the trench, wherein a portion of the ILD in the trench forms asecond STI region.

In accordance with yet another aspect of the present invention, anintegrated circuit structure includes a semiconductor substrateincluding an active region; a first STI region adjoining a first side ofthe active region; and a MOS device. The MOS device includes a gatestack on the active region and the first STI region; a source/drainregion adjacent the gate stack and in a portion of the active region;and a source/drain silicide region on the source/drain region. Theintegrated circuit structure further includes a second STI regionadjoining a second side of the source/drain region perpendicular to thefirst side, wherein first top surfaces of first portions of thesource/drain region and the source/drain silicide region adjoining thesecond STI region are flatter than second top surfaces of secondportions of the source/drain region and the source/drain silicide regionadjoining the first STI region.

In accordance with yet another aspect of the present invention, anintegrated circuit structure includes a semiconductor substrateincluding a first and a second active region; a first STI region in thesemiconductor substrate, wherein the first STI region has a firstlengthwise direction, and wherein the first STI region adjoins the firstand the second active regions; a first gate electrode strip over thefirst active region and the first STI region; a second gate electrodestrip over the second active region and the first STI region, whereinthe first and the second gate electrode strips are substantiallyparallel to each other; a first source/drain region in the first activeregion and adjacent the first gate electrode strip; a first source/drainsilicide region on the first source/drain region; a second source/drainregion in the second active region and adjacent the second gateelectrode strip; a second source/drain silicide region on the secondsource/drain region; and an ILD having a top surface higher than topsurfaces of the first and the second gate electrode strips, wherein theILD extends into the trench to form a second STI region adjoining thefirst and the second source/drain regions.

An advantageous feature of the present invention is that undesirablestresses to MOS devices are reduced, particularly in the gate lengthdirection of the MOS devices. The performance of the MOS devices maythus be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a top view of a conventional integrated circuitstructure, wherein metal-oxide-semiconductor (MOS) devices are isolatedfrom each other by shallow trench isolation regions;

FIGS. 2A through 8 are cross-sectional views and top views ofintermediate stages in the manufacturing of embodiments of the presentinvention;

FIG. 9 illustrates a top view of an integrated circuit structure,wherein shallow trench isolation (STI) regions have irregular shapes;and

FIG. 10 illustrates an NMOS embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A novel method for forming shallow trench isolation (STI) regions andthe resulting structures are provided. The intermediate stages ofmanufacturing preferred embodiments of the present invention areillustrated. The variations of the preferred embodiments are thendiscussed. Throughout the various views and illustrative embodiments ofthe present invention, like reference numbers are used to designate likeelements.

FIGS. 2A and 2B illustrate cross-sectional views of semiconductor chip18, which include semiconductor substrate 20. In an embodiment, asillustrated in FIG. 2A, semiconductor substrate 20 is formed of a bulksemiconductor material such as silicon. In alternative embodiments, asillustrated in FIG. 2B, semiconductor chip 18 has asemiconductor-on-insulator (SOI) structure, with semiconductor substrate20 located on a buried oxide (BOX) layer 22, which is further located onanother semiconductor layer 24.

FIG. 3A illustrates a top view of the formation of STI regions 28 insemiconductor chip 18. In the illustrated region of semiconductor chip18, two parallel STI regions 28 are formed. FIG. 3B illustrates across-sectional view of the structure shown in FIG. 3A, wherein thecross-sectional view is taken along a plane crossing line A-A′ in FIG.3A. An exemplary formation process of STI regions 28 is brieflydiscussed as follows with reference to FIG. 3B. First, pad layer 34 andhard mask layer 36 are formed on substrate 20, followed by the formationof trenches (the spaces occupied by STI regions 28). The trenches may beformed by anisotropic plasma etching, for example, usingfluorine-containing chemicals. In the case semiconductor chip 18 has anSOI structure, the trenches may have a depth equal to the depth of thesemiconductor substrate 20, so that the subsequently formed STI regions28 physically contact BOX layer 22.

Next, trench liners 30 are formed in the trenches, using thermaloxidation. Accordingly, trench liners 30 may include silicon oxide,although other dielectric materials may also be used. Next, dielectricmaterial 32 is filled into the remaining portion of the trenches. Thefilling of dielectric material 32 may be performed using high-densityplasma chemical vapor deposition (HDPVD). However, other commonly usedmethods such as high aspect-ratio process (HARP), sub-atmospheric CVD(SACVD) and spin-on can also be used. A chemical mechanical polish (CMP)is then performed to remove excess dielectric material 32 over hard mask36. Next, hard mask 36 and pad layer 34 are removed, leaving STI regions28 in the trenches.

Next, as shown in FIGS. 4A and 4B, gate electrodes 38 (and underlyinggate dielectrics, not shown) are formed, wherein gate electrodes 38 formportions of the resulting MOS devices 40 and 42. In an exemplaryembodiment as shown in FIG. 4B, MOS devices 40 and 42 are PMOS devices.In alternative embodiments, as shown in FIG. 10, MOS devices (which arerenamed as MOS devices 140 and 142) are NMOS devices. In yet otherembodiments, MOS devices 40 and 42 include one PMOS device and one NMOSdevice. MOS device 40 includes gate 44 (a portion of one of the gateelectrodes 38), source/drain regions 46 and 48 (extending fromrespective source/drain silicides 50 and 52 to BOX layer 22),source/drain silicide 50 and 52, and optional silicon germanium (SiGe)stressors 47 and 62. MOS device 42 includes gate 54 (a portion of one ofthe gate electrodes 38), source/drain regions 48 and 56, source/drainsilicides 52 and 60, and optional SiGe stressors 57 and 62. MOS devices40 and 42 share common source/drain region 48, source/drain silicide 52,and SiGe stressor 62. Gate silicides 45 and 55 may also be formed ongates 44 and 54, respectively. The formation details of MOS devices 40and 42 are known in the art, and hence are not repeated herein. As isknown in the art, the formation of MOS devices 40 and 42 may includeseveral high-temperature processes, such as the activation ofsource/drain regions 46, 48, and 56, and the formation of silicideregions 50, 52, and 60. During the high-temperature steps, thetemperatures may be as high as about 700° C.

Referring to FIG. 5, after the high-temperature steps have beenperformed, trenches 64 are formed to cut into substrate 20. Trenches 64are perpendicular to STI regions 28 in a top view (refer to FIG. 6B,wherein STI region 74 is formed in trenches 64). In the casesemiconductor chip 18 has the SOI structure, trenches 64 extend deepenough, so that BOX layer 22 is exposed. In the case semiconductor chip18 is a bulk substrate, trenches 64 extend to below the bottoms 49 ofsource/drain region 48, for example, to a depth comparable to that ofSTI regions 28. One of the trenches 64 separates each of the commonlyshared source/drain region 48, source/drain silicide 52, and SiGestressor 62 to a first portion (denoted with subscript “1”) belonging toMOS device 40, and a second portion (denoted with subscript “2”)belonging to MOS device 42. Since trenches 64 are formed after theformation of source/drain region 48, source/drain silicide 52, and SiGestressor 62, the top surfaces of source/drain regions 48 ₁ and 48 ₂ andsource/drain silicide regions 52 ₁ and 52 ₂ may each include an edgeportion substantially flat, particularly the edge portions in a middleposition between two STI regions 28 (refer to FIG. 6B). As a comparison,the portions of source/drain regions 48 ₁ and 48 ₂ and source/drainsilicide regions 52 ₁ and 52 ₂ adjoining STI regions 28 are less flat,for example, with a profile as shown in FIG. 6C, which is across-sectional view taken along a plane along line C-C′ in FIG. 6B.

FIG. 6A illustrates the formation of contact etch stop layer (CESL) 66,inter-layer dielectric (ILD) 70, and contact plugs 72. Contact etch stoplayer 66 is first blanket formed, followed by the formation of ILD 70.CESL 66 may be formed of silicon nitride or other materials such assilicon oxy-nitride. As is known in the art, CESL 66 may apply a stressto the underlying MOS devices 40 and 42 to improve the carrier mobility.ILD 70 may include silicon oxide formed using, for example, SACVD.Alternatively, ILD 70 may include boronphosphosilicate glass (BPSG),phosphosilicate glass (PSG), or the like. Relatively low-temperaturesare needed for the formation of CESL 66 and ILD 70, wherein thetemperatures may be about 450° C. or lower, for example, about 400° C.CESL 66 and ILD 70 extend into trenches 64. Throughout the description,the portions of CESL 66 and ILD 70 underlying the top surface of siliconsubstrate 20 are referred to as STI regions 74. In alternativeembodiments, before the formation of ILD 70, a dielectric material maybe filled into trenches 64 to form STI regions 74, followed by theformation of CESL 66 and ILD 70. In this case, the dielectric materialin trench 64 may be different from that of ILD 70.

FIG. 6B illustrates a top view of the structure shown in FIG. 6A, andthe cross-sectional view shown in FIG. 6A is taken along the planecrossing line B-B′ as shown in FIG. 6B. The top view shows that gateelectrode strips 38 crosses over STI regions 28, while STI regions 74 donot cross any gate electrode strip.

Next, as also shown in FIG. 6A, openings (occupied by contact plugs 72)are formed in ILD 70 and CESL 66 to expose source/drain silicide regions50, 52, and 60, and the silicide regions 45 and 55. Contact plugs 72 arethen formed in the openings. As is known in the art, contact plugs 72may include tungsten.

FIGS. 7A and 7B illustrate further cross-sectional views taken along aplane crossing line D-D′ as shown in FIG. 6B. FIG. 7A illustrates theembodiment formed on an SOI substrate. The bottoms of STI regions 28 andthe bottoms of STI regions 74 (including CESL 66) all extend to BOXlayer 22, and hence are leveled to each other. FIG. 7B illustrates STIregions 28 formed in bulk substrate 20. In this case, the bottoms 80 ofSTI regions 28 may be higher than, substantially leveled with, or lowerthan, the bottoms 78 of STI regions 74. Further, it is realized that thebulk of STI regions 28 and 74 (excluding the STI liners and CESL 66) maybe formed of a same material, for example, silicon oxide, or differentmaterials. Also, the bulk of STI regions 28 and 74 may be formed usingdifferent formation methods even if they are formed of a same material.In this case, STI regions 28 and 74 may still be distinguished from eachother because of their different characteristics. In an exemplaryembodiment, the bulk of STI regions 74 have a lower density than STIregions 28.

FIG. 8 illustrates an alternative embodiment of the present invention.In this embodiment, the formation and the filling of trenches 64 (referto FIG. 5) is performed after the formation of CESL 66, but before theformation of ILD 70. Accordingly, CESL 66 is etched during the formationof trenches 64, and hence the resulting STI regions 74 do not includeCESL 66.

Referring to FIG. 9, STI regions 28 and/or STI regions 74 may not formrectangular patterns, and the MOS devices may not be fully surrounded bythe STI regions. However, the same teaching applies, and the portions ofSTI regions having no gate electrode strips formed thereon may be formedusing the same methods, and formed simultaneously as, STI regions 74,while the portions of STI regions having gate electrode strips formedthereon may be formed using the same methods, and formed simultaneouslyas, STI regions 28. Alternatively, the portions of STI regions havinglengthwise directions parallel to the source-to-drain direction of theadjacent (or adjoining) MOS devices may be formed using the samemethods, and formed simultaneously as, STI regions 28, while the portionof STI regions having lengthwise directions not parallel to (forexample, perpendicular to) the source-to-drain direction of the adjacent(or adjoining) MOS devices may be formed using the same methods, andformed simultaneously as, STI regions 74. Since STI regions 74 typicallyhave smaller widths than the widths of STI regions 28, voids or othertypes of defects may be formed in STI regions 74. However, since STIregions 74 are formed after the high-temperature steps have beenperformed, the defect will not cause adverse stresses in the directionparallel to the source-to-drain direction of MOS devices 40 and 42.

FIG. 10 illustrates yet another embodiment of the present invention. Inthis embodiment, NMOS devices 140 and 142 are formed on a same substrate20 as PMOS devices 40 and 42. Again, STI regions 74 may be formed usingthe same method, and formed simultaneously, as the formation of STIregions 74 as shown in FIGS. 6A and 6B. In alternative embodiments, onetype of MOS devices, such as PMOS devices are surrounded by STI regions28 and 74, as is shown in FIG. 6B, while on the same substrate 20 (inthe same semiconductor chip 18), another type of MOS devices, such asNMOS devices, are surrounded by STI regions 28 only. This embodiment maybe used if the stresses applied to the source-to-drain direction of theadjacent (or adjoining) MOS devices are beneficial for the performanceof the MOS devices.

The embodiments of the present invention have several advantageousfeatures. By adopting a two-step formation of STI regions, the adversestresses applied to the MOS devices adjacent to the STI regions arereduced. The fluctuation in the dopant concentration is also reduced.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method of forming an integrated circuit structure, the methodcomprising: providing a semiconductor substrate; forming a firstisolation region in the semiconductor substrate; after the step offorming the first isolation region, forming a metal-oxide-semiconductor(MOS) device at a surface of the semiconductor substrate, wherein thestep of forming the MOS device comprises forming a source/drain region;and after the step of forming the MOS device, forming a second isolationregion in the semiconductor substrate comprising: after the step offorming the source/drain region, etching a portion of the source/drainregion away from a gate spacer of the MOS device to form a trench; andforming an inter-layer dielectric (ILD), wherein the ILD extends intothe trench to form the second isolation region.
 2. The method of claim1, wherein the second isolation region has a bottom no higher than abottom of the source/drain region.
 3. The method of claim 1, wherein thefirst isolation region adjoins a first side of the source/drain region,and the second isolation region adjoins a second side of thesource/drain region.
 4. The method of claim 3, wherein the first side isparallel to a source-to-drain direction of the MOS device, and thesecond side is perpendicular to the source-to-drain direction of the MOSdevice.
 5. The method of claim 1 further comprising a buried oxide layerunderlying and adjoining a bottom surface of the semiconductorsubstrate, wherein the first and the second isolation regions bothcontact the buried oxide layer.
 6. The method of claim 1, wherein thesemiconductor substrate is a bulk substrate, and wherein the first andthe second isolation regions extend into different depths of thesemiconductor substrate.
 7. The method of claim 1 further comprising:forming a source/drain silicide region on the source/drain region; andblanket forming a contact etch stop layer (CESL) between the ILD and thesource/drain silicide region, wherein the step of forming the trench isperformed after the step of blanket forming the CESL.
 8. The method ofclaim 1 further comprising: forming a source/drain silicide region onthe source/drain region; and blanket forming a CESL between the ILD andthe source/drain silicide region, wherein the step of forming the trenchis performed before the step of blanket forming the CESL.
 9. A method offorming an integrated circuit structure, the method comprising:providing a semiconductor substrate; forming a metal-oxide-semiconductor(MOS) device comprising: forming a gate stack on a top surface of thesemiconductor substrate; forming a gate spacer on a sidewall of the gatestack; forming a source/drain region adjacent the gate spacer; andforming a silicide region on the source/drain region; etching a portionof the silicide region and a portion of the source/drain region to forma trench, wherein the trench is spaced apart from an outer edge of thegate spacer by a remaining portion of the source/drain region, andwherein the trench extends below a bottom of the source/drain region;and forming an inter-layer dielectric (ILD) over the MOS device, whereinthe ILD extends into the trench to form a shallow trench isolation (STI)region.
 10. The method of claim 9 further comprising, before the step offorming the trench, forming a contact etch stop layer (CESL) between thesilicide region and the ILD, wherein the step of forming the trenchcomprises etching a portion of the CESL.
 11. The method of claim 9further comprising, after the step of forming the trench, forming a CESLbetween the silicide region and the ILD, wherein the CESL extends intothe trench.
 12. The method of claim 9, wherein the STI region adjoins afirst side of the source/drain region, and wherein the method furthercomprises forming an additional STI region before the step of formingthe MOS device.
 13. The method of claim 12, wherein the additional STIregion adjoins a second side of the source/drain region, and wherein thesecond side is perpendicular to the first side.
 14. The method of claim12, wherein the STI region and the additional STI region comprise a samedielectric material, and wherein the steps of forming the additional STIregion and forming the ILD are performed using different methods. 15.The method of claim 12, wherein the STI region and the additional STIregion comprise different dielectric materials.
 16. The method of claim9, wherein the STI region extends down to contact a buried oxide layerunderlying the semiconductor substrate.
 17. A method of forming anintegrated circuit structure, the method comprising: providing asemiconductor substrate; forming a first shallow trench isolation (STI)region in the semiconductor substrate, wherein the first STI region hasa first lengthwise direction, and wherein the first STI region adjoinsan active region; forming a first and a second gate electrode strip overthe active region and the first STI region, wherein the first and thesecond gate electrode strips are parallel to each other; forming acommon source/drain region in the active region and between the firstand the second gate electrode strips; forming a silicide region on thecommon source/drain region; etching a portion of the silicide region anda portion of the common source/drain region to form a trench, whereinthe trench has a second lengthwise direction parallel to the first andthe second gate electrode strips; and forming an inter-layer dielectric(ILD) having a top surface higher than top surfaces of the first and thesecond gate electrode strips, wherein the ILD extends into the trench toform a second STI region.
 18. The method of claim 17 further comprising,before the step of forming the trench, forming a contact etch stop layer(CESL) on the silicide region, wherein the step of forming the trenchcomprises etching a portion of the CESL.
 19. The method of claim 17further comprising, after the step of forming the trench, forming a CESLon the silicide region, wherein the CESL extends into the trench.